Question

What do the following indicators tell you about whether a reaction can proceed as written? (a) The standard free-energy change is positive. (b) The free-energy change is positive. (c) The reaction is exergonic.

Short answer

A positive ΔG or ΔG° means the reaction cannot proceed as written. An exergonic reaction can proceed.

Step-by-step solution

- Understanding the Standard Free-Energy Change

The standard free-energy change (ΔG°) tells us about the spontaneity of a reaction under standard conditions (1 atm, 25°C, 1 M concentration). If ΔG° is positive, the reaction is not spontaneous under standard conditions and will not proceed as written.

- Interpreting a Positive Free-Energy Change

The free-energy change (ΔG) determines the spontaneity of a reaction under the specific conditions of the reaction. If ΔG is positive, the reaction is non-spontaneous under the given conditions and will not proceed as written.

- Understanding Exergonic Reactions

An exergonic reaction is one where ΔG is negative. This indicates that the reaction is spontaneous and can proceed as written. Exergonic means energy-releasing.

Key concepts

Standard Free-Energy Change

The standard free-energy change, denoted as \(\Delta G^\circ\), is a measure of the energy change during a reaction under standard conditions which are defined as 1 atm pressure, 25°C temperature, and 1 M concentration for all reactants and products. This value gives us insights into the spontaneity of a reaction under these standard conditions.
If \(\Delta G^\circ\) is positive, it means that the reaction requires an input of energy to proceed, and it is not spontaneous under these specific conditions. In other words, the reaction won't happen by itself and external energy is needed to drive the reaction forward. Conversely, if \(\Delta G^\circ\) is negative, the reaction is spontaneous under the standard conditions and will proceed without the need for additional energy.
Understanding the standard free-energy change is crucial for predicting how reactions behave in laboratory and real-world settings. It helps chemists and biochemists design reactions that are feasible and efficient.

Spontaneity of Reactions

The spontaneity of a reaction is determined by the free-energy change, denoted as \(\Delta G\). Unlike the standard free-energy change, \(\Delta G\) takes into account the actual conditions under which a reaction occurs, not just the standard conditions.
If \(\Delta G\) is positive, the reaction is non-spontaneous under the given conditions. This means the reaction will not proceed on its own and requires additional energy to make it happen. Non-spontaneous reactions can be driven by coupling them with spontaneous reactions or by supplying external energy sources such as ATP in biochemical processes.
On the other hand, if \(\Delta G\) is negative, the reaction is spontaneous under the existing conditions and will proceed without the need for extra energy. Notably, spontaneity does not indicate the speed of the reaction; it simply means that the reaction is thermodynamically favorable.

Exergonic Reactions

Exergonic reactions are reactions where the free-energy change, \(\Delta G\), is negative. This negative value indicates that these reactions release free energy as they proceed, making them spontaneous.
The term 'exergonic' is derived from 'exo-' meaning 'outwards' and 'energy', hence it literally means energy-releasing. These reactions are crucial in biological systems as they often provide the necessary energy for endergonic (energy-consuming) reactions. For example, the breakdown of glucose in cellular respiration is an exergonic process that provides the energy needed for various cellular activities.
In summary, understanding exergonic reactions is essential for grasping how energy flows in biochemical pathways and how cells harness energy to perform functions vital for life.